The exhaust gas purifier such as a diesel oxidation catalyst (DOC), a diesel particulate filter (DPF), a selective catalytic reduction catalyst (SCR) fully functions once a certain temperature environment or above is established. For instance, FIG. 18 is a table representing relationships among a temperature of a SCR catalyst, NOx concentration at an inlet and an outlet of the SCR and an operation time of the engine in the SCR unit for reducing nitrogen oxides (NOx). As illustrated in FIG. 18, the NOx concentration at the outlet of the SCR is relatively high until the catalyst temperature of the SCR unit reaches its activating temperature, and the SCR unit does not fully demonstrate the NOx purification function. Therefore it is necessary to raise the temperature of the exhaust gas purifier such as the SCR unit at an early stage during the warm-up operation such as immediately after starting the engine, in order to perform the exhaust gas purification at an early stage.
In order to raise the temperature of the exhaust gas purifier at the early stage, it is known to change a fuel injection timing, or to control (reduce) a flow rate of the air to be supplied to the engine by controlling a variable supercharger and an air supply throttle. For instance, disclosed in Patent Literature 1 is an example of controlling the fuel injection timing. Disclosed in Patent Literature 2 is an example of controlling the flow rate of the air to be supplied to the engine.